1. Field of the Invention
The present invention relates to a projection type color image display unit which composes three images transmitted through three optical shutters corresponding to three primary colors to obtain a color image.
2. Description of the Related Art
The display unit is roughly classified into a direct type and a projection type, and in case of a large screen display, the projection type is superior to the direct type. There are a variety of the projection type display units one of which is a system using the optical shutter where devices are relatively compactly received therein. The optical shutter has an image display capability, and normally includes pixels in a matrix form. A display unit having a circuit structure of an active matrix type in which a switching element is disposed at each pixel is excellent in contrast. However, a passive matrix type display unit (a simple matrix type) is also used. Further, to shutter a light, a method in which liquid crystal material is used, and the light is shuttered by a voltage or current which is applied to the liquid crystal is simple. It is needless to say that other methods have also been proposed.
Displaying a color image is normally conducted by separating the color image into three color components and composing separated colors. Therefore, three optical shutters are required. The conventional projection type color image display unit is designed as shown in FIGS. 2A and 2B.
In the display unit shown in FIG. 2A, three white lights that have passed through optical systems 2 to 4 from one light source 1 are inputted to optical shutters 8 to 10 through color filters 5 to 7. In other words, those three color filters are red, blue and green, respectively, and the optical shutters corresponding to the respective color filters display images corresponding to the respective primary colors.
Lights that have passed through the optical shutters are converged into one light through optical systems 11 to 13 to produce a color image. This color image is projected onto a screen (FIG. 2A).
In this system, since ⅔ of the incident lights are absorbed by the color filters at the minimum, the light from the light source is not sufficiently utilized with the result that the image becomes dark. In order to solve this problem, a dichroic mirror is employed, and an example thereof is shown in FIG. 2B. U.S. Pat. No. 5,337,171 discloses such a projector, the content of which is incorporated herein by reference.
First, of a white light inputted to a dichroic mirror 14 from a light source 1, lights other than a red light (lights shorter in wavelength than a red light) are reflected, and only the red light is inputted to an optical shutter 10. The remaining lights are inputted to a second dichroic mirror 15. In this example, lights other than a green light (lights shorter in wavelength than a green light), that is, a blue light is reflected and then inputted to an optical shutter 9. A light which are linearly propagated through the second dichroic mirror 15 is green, which is inputted to an optical shutter 8 through a total reflection mirror 16. The lights that have passed through the respective optical shutters are converged into one light through optical systems 11 and 12 to produce a color image. This color image is projected onto a screen (FIG. 2B).
In this system, since no light is absorbed by the color filters, the light can be efficiently utilized. In order to enhance the efficiency, a light from the light source may be an intense light consisting of the spectra of three primary colors. With this, the transmission and reflection of a light at the dichroic mirror are efficiently conducted.
Also, there has been proposed a method in which a red laser 21, a green laser 22 and a blue laser 23 are employed as light sources, and lights which are expanded by beam expanders 24 to 26 are made input to optical shutters 27 to 29, as shown in FIG. 3.
A block diagram of a picture image circuit in the projection type display unit of this type is shown in FIG. 4. A video signal is information of three primary colors which are formed on a carrier wave through the AM-FM composite modulation system. The signal is separated into three primary color signals through demodulation. The demodulation operation is conducted by a “color separation” circuit. At this stage, the respective primary color signals are analog signals, but they are digitalized into digital signals by a subsequent digitalizing circuit (an analog-to-digital converter, an A/D circuit). Then, the respective primary color signals are transmitted to three optical shutters to be subjected to a γ-correction at the circuits provided on the respective optical light shutters. Thereafter, the digital signals are converted into analog signals by a subsequent digital-to-analog circuit (a digital-to-analog converter, a D/A circuit), and then inputted to a display unit (an optical shutter).
The color image display unit with the above structure requires a chip having a color separation circuit and an A/D circuit in addition to those three optical shutters. In other words, at least four parts for constituting the circuit are required at the minimum.
Also, there arises such a problem that in the color separation circuit or the A/D circuit, when a noise is intermingled therein, it cannot be finally removed therefrom.